Condenser microphone for frequency modulating oscillating circuit



July 13, 1954 o. VIERLING EIAL 2,683,861 CONDENSER MICROPHONE FOR FREQUENCY MODULATING OSCILLATING CIRCUIT Filed NOV. 22, 1950 3 Sheets-Sheet 1 i lnvenlo rs 05w? V/lsw W6 y 1954 o. VIERLING AL 2,683,861

DEN-SEER MICRO NE FREQU Y ODULATING 0S LATING CIRCU Filed Nov. 22, 1950 3 Sheets-Sheet 2 In vs 0 to PS 0"? V/ERL m/s ZuoW/G Sc/w/mLE/e July 13, 1954 o. VIERLING ETAL 2,683,861 CONDENSER MICROPHONE FOR FREQUENCY MODULATING OSCILLATING CIRCUIT Filed NOV. 22 1950 3 Sheets-Sheet 3 30 In venfors 054471? V/f/PLMG ZOow/a Scam 01.51?

Patented July 13, 1954 Oskar Vierling, Oberfranken,

Schindler, Forchheim,

Germany Bayern, and Ludwig Oberfranken, Bayern,

Application November 22, 1950, Serial No. 196,984

Claims priority, application Switzerland April 22, 1950 19 Claims.

It is known in principle to impress upon a high frequency oscillating circuit the pattern of an acoustic signal by means or" a condenser microphor. However, the arrangements heretolore used were impractical, because condenser microphones of the usual construction have been These microphones do not meet the requirements of faultless reception of lower freouencies, suificient frequency amplitude, and good maintenance of the middle frequency; and they lack the mechanical sensitivity of construction needed especially for use outdoors.

The prim ry object of this invention is to provide an oscillating circuit which is particularly suited for the frequencies of the ultra short wave ranges and in which the frequency modulation is eiiected by means of a condenser microphone connected the oscillating circuit. For this purpose, there is provided a special construction of such a microphone and a new circuit for connectin such a special type of microphone to an ante or the like. The frequency modulated oscillator can then be used either directly as the transmitter for the emission of ultra short waves, or as an oscillating circuit which serves, after subse uent demodulation and amplification of the lower frequencies, as a novel and simple connection of a condenser microphone for such puras, for instance, a public address system or a hearing aid.

The condenser microphone, according to this inve ition, which is to effect the frequency modulation contains at least two electrodes situated at a very small distance from the membrane, these electrodes being preferably arranged symmetrically to each other, and forming partial capacitances opposite the membrane. These partial capacitances directly or indirectly influence the frequency of the oscillating circuit when their values are changed. Preferably, the two capacitances should be connected in series in the oscillating circuit, because by connecting them in that way, the total capacitance becomes smaller, which is of particular advantage in ultra short wave circuits in view of the high frequency to be attained.

The electrodes of the microphone, which preferably consist of substantially semicircular symmetrically arranged metal parts and which are situated next to the membrane, are fastened on an insulating plate preferably made of some ceramic material having a small angular loss. n the opposite side of the insulating plate, away from the membrane, there are electrically conducting coatings, each of which forms a capaci-.

tance with the electrode situated opposite it. These capacitances thus formed influence the oscillating circuit, forming for example a feedback capacitance for producing a feedback potential which actuates and maintains the oscillation.

The electrodes are preferably soldered on to a silver coated and burnt-in coating. They may also be constructed as concave electrodes. In some cases it may be suitable not to make them in the form of concave electrodes, and to attach them from the rear by means of screws. In that case, there should be no conductive coatings on the reverse side of the insulating plates, because the screws penetrate through the plate. It is then possible to use condensers connected to the electrodes in order to produce the feedback.

In order to make sure that each of the electrodes fastened on to the ceramic material coacts only With the corresponding coating and to prevent the jumping of dielectric displacement currents to other coatings, these coatings are separated from each other by means of strips of conductive material which are kept at zero potential. If there are only two such electrodes present, then a single strip is sufficient to prevent any possible diagonal flow of displacement currents.

By using a condenser microphone with the above described special properties it is possible to use many simple and effective connections for high frequency oscillating circuits, in which the frequency modulation is effected by means of the condenser microphone.

A few of the particularly suitable connections for such oscillating circuits for condenser. microphones with two electrodes are described in the following paragraphs. However, the invention is not confined to these particular connections; there are numerous possible variations of the connections described herein, in which the advantages of the new condenser construction may also be brought out.

In a particularly simple arrangement, the condenser electrodes, which are in the form of two segments of the same circle, are connected with two oscillator tubes, which are operated in counter-phase, and with the oscillating circuit in such a manner that the two electrodes are connected to the respective ends of the self-inductance of the oscillating circuit and that the electrodes are connected to the respective anodes of the oscillating tubes. The grids of these tubes are cross-connected with the coatings that lie opposite the electrodes of the microphone, so that the tubes receive the necessary feedback potenmicrophone are connected with the ends of the self-inductance of the oscillating circuit. In addition, one of the two electrodes is connected to the anode of one oscillator tube. One of the coatings in this circuit is connected to the grid of the oscillator tube and the two of them are connected respectively to grids of tubes in the amplifying stage which operate in counter-phase.

For this method of connection only one iductance and two-resistancesare needed besides the three tubes.

Whereas, in the usual condenser microphones heretofore used the full anode potential was imposed between the membrane and the counterelectrode, and thus great danger of contact between them was created by the static attraction forces, this disadvantage can be largely eliminatcd in the oscillating circuit described in this invention, if the microphone with two electrodes is used in a suitable circuit. For instance, it is possible to assemble a transmitting circuit in;

which there is imposed between the microphone membrane and the electrodes only the continuone current voltage of the tubegrids. In such a circuit, the distance between the membrane and the electrodes can be reduced to a minimum and theefiectiveness of the soundwaves on the oscillating circuit can be considerably increased, because with such a small distance between the membranes and the electrodes, the vibrations of the membrane result in considerable changes of capacitance in the oscillating circuit. In order to effect such an arrangement, the electrodes of the condenser microphone are crossed with the grids of oscillator tubes that operate in counterphase, and the coatings are connected each to an anode of one of the tubes,'and each also to one end of the self-inductance of the oscillating circuit. An oscillating circuit connected in this manner has proved to be particularly valuable as a means of frequency modulation, because of the substantial reduction in the distance between membrane and electrodes.

It is even possible to avoid completely a continuous current voltage between membrane and electrode by changing the construction of the microphone slightly. For this purpose, there are added to the microphone, between the electrodes and the coatings, additional conducting plates which are preferably insulated by means'of ceramic parts.

the circuit as capacitancesas has already been The electrodes are then coupled to described, whereas-the conductive plates take the part of the electrodes. Thus, there is no danger of a short circuit between membrane and electrodes because of the static forces of attraction due to the continuous current voltage, so that the electrodes can be brought as closely to the membrane as the vibration amplitude of the membrane will permit.

The simplicity of construction of the disclosed oscillating circuit with reference to the small number of parts necessary therefor, allows the arrangement of said parts so that the base on which the individual parts are mounted, and the parts themselves, can-be accommodated in a hollow cylinder only a few centimeters in diameter. The base also carries the condenser microphone and an insulating end plate in such a Way that when the base is introduced into the hollow cylinder, the latteris closed off on one end by the microphone and on the other by the end plate.

The entire circuit takes up so little room after it is introduced into the hollow cylinder that it may easily be grasped with one hand, and that its overall length is not greater than the width of the back of an average hand.

The base may be suitably subdivided into three compartments, in one of which the tubes are mounted parallel with the axis of the cylinder,

-- whereas another compartment holds the selfinductances which are mounted preferably perpendicular to the axis of the cylinder, and the middle compartment contains the electrical connections between the parts in the other two. The tubes, accordingto this invention, are held in position only by the current-carrying wires and by their tops which protrude into small holes in the insulating plate at one end of the cylinder.

In order to make the necessary adjustments on theoscillating circuit after it is introduced into the cylinder, the latter is provided with openings through which the tools necessary to make changes in the values of the self-inductance can be introduced. Similarly, for purposes of introducing the anode voltage from the antenna, an opening which continues to the edge of the cylinder can be provided, unless such a connection is provided at the bottom of the cylinder.

The construction of the condenser microphone necessary for the oscillating circuit, suitable for the invention, should be such that there is a holding ring provided for the ceramic insulating body of the microphone. The ceramic material can be either glued in, soldered in or held in position by means of a pressure plate. Between the holding ring and the protective cover of the microphone there is also provided an intermediate ring. Furthermore, between the holding 'ring and the intermediate ring there is inserted a spacing ring which maintains the desired distance between the electrodes and the membrane. Since this spacing ring is interchangeable, the distance between the membrane and the electrode, which is set at zero without the ring, can

be varied to within of a millimeter with great'accuracy. However, it has proved to be best suitable to maintain a distance of from .007

'to .02 millimeter.

The membrane is stretched between the intermediate ring and the cover of the microphone. In order to facilitate this stretching and to obtain a tightly stretched membrane, the plane side of the microphone cover which touches the membrane is provided with several round protrusions which are arranged in a circle, and the intermediate ring has at least one well-like recess into which'th'e protrusions will fit tightly. When the two plates are pressed together the member will stretch automatically toward the edges. The holding ring, the intermediate ring and the microphone cover are held together by means of a screw thread/For manufacturing reasons it is under certain circumstances desirable to pro vide both sides or only one side with well-like recesses. The result is then that, once the membrane is stretched, it cannot change its stretched 75 position.

In order to compensate for the frequency var ations of the transmitter caused by the heating of the tubes, we propose to screw the ceramic plate into the microphone holder, depending on the requirements, up to a certain elevation of its edge. The variations in length which occur as a result of temperature changes, and as they affect the positions of the various members and the electrodes, must produce a, change in the distance of the membrane from the electrodes, such that they compensate for the change in frequency of the oscillating circuit by varying the distance between the electrodes and the memcrane to such an extent that the frequency remains the same.

Since the frequency also depends upon the anode voltage which changes with the degree of use of the batteries, this invention also compensates for this frequency variation. For this purpose the electrodes which lie opposite the membrane are connected to a continuous current voltage, so that when this voltage decreases, the distance from the membrane increases, whereby the capacity becomes smaller and the change in ircouency due to the voltage properties of the is equalized. Even under these circumstances we have very favorable conditions, for "stance, at a membrane clearance of .02 mm. -.nd at 50 volts we have a critical point, so that changes in the anode voltage of between 40 and so volts leave the system unaffected.

According to a further feature of this invenion, the ceramic insulating piece is made of a ."Vramic material which possesses a dielectric constant as low as possible in order to make the capacitance between the electrodes as small as possible.

For the same reason, the electrodes may be formed so that the distance between them inas the d stance from the membrane inor so that only the upper end of the odes form semicircles, whereas the remainin parts of the electrodes are more distant from c other, although that distance is constant. small distance between the membrane and the electrodes in the condenser microphone l necessary to afford the layer of air een the two a means of escape, so that the cushion will not exert a restraining action upon the movements of the membrane. In order to avo d such an action this invention provides r -pass for the air by means of perpendicular holes which are distributed all over the top of the electrodes.

i order to equalize gradual pressure changes wii may occur as a result of changes in atmospr pressure or variations in temperature, a

1 hole is provided which transmits an equalpressure from the outside to the inside of the microphone.

Th protective covering of the microphone is with circular openings which are covby a screen to prevent the entry and accucenter of the membrane may be covered by a protective cover attached to the screen, so that this portion of the membrane is protected from the excessively strong action of the sound waves. Such a protective covering has proved to be advantageous to produce a true frequency modu- V-.. This cover may also serve as an insulating body for the antenna plug.

Despite great care in the construction of condensers and coils it is impossible to buildthem absolutely symmetrical. In order to .minimize the effects of such unavoidable diosymmetry, the lead wire to the center connection of the inductance coils is provided with a resistance which has a value of about 2000 ohms.

Further objects and advantages of the invention will appear more fully from the following description, especially when taken in conjunction with the accompanying drawings which form a part thereof.

In the drawings:

Fig. 1 is a circuit diagram of a frequency modulated ultra short wave oscillating circuit with a specially built condenser microphone, with amplification stage and antenna;

Fig. 2 is a diagram of an oscillating circuit with a three-point connection and amplification stage;

Fig. 3 shows another oscillating circuit with only small constant current voltage between membrane and electrodes of the condenser microphone;

Fig. t shows a circuit in which there is no constant current voltage between the membrane and. the electrodes of the condenser microphone;

Fig. 5 shows in perspective the mounting of the oscillating circuit and condenser microphone assembly;

Fig. 6 is a perspective view of the hollow cylinder or cover for the assembly as shown in Fig. 5, but on a smaller scale;

Fig. 7 is a sectional View through the center of the condenser microphone;

Fig. 8 is a bottom view thereof;

Fig. 9 is a cross-section along the line IX-IX of Fig. 7;

Figs. 10 and 11 are partial longitudinal sections through the microphone showing modificaticns thereof; and

Fig. 12 shows in perspective a container for the oscillating circuit and the necessary batteries.

The condenser microphone 28 used in these circuits, and which forms a part of the invention, consists of the following individual parts that are held together by means of a container:

(a) A membrane I maintained at zero potential.

(b) The electrodes 2 and 2 formed as segments of the same circle and with their upper faces closely approaching the membrane I.

(0) Two coatings d and 4 insulated from elcctrodes 2 and 2 by a ceramic insulating plate 8 and positioned opposite said electrodes.

(d) An electrically conductive strip 5 lying between coatings 3 and 4 which is maintained at zero potential.

In the circuits as represented by Figs. 1 through 4, the insulating plate 3 which lies between the electrodes 2 and 2 and the coatings 3 l is not shown. All the circuits shown in those drawings have one thing in common, namely that they represent oscillating circuits which are frequency modulated by a specially built condenser microphone, and thus lend themselves especially well to the generation of ultra short waves.

The oscillating circuit shown in Fig. 1 consists of two oscillator tubes 6, T1 which work in counterphase and whose anodes 8, 9 are connected to the ends of the self-inductance it. The center ii of the self-inductance is supplied with the anode voltage from a suitable source preferably through a resistance 62 of about 2000 ohms for the purpose of avoiding the disturbing effect of asymmetry. The partial capacitances consisting of the membrane I and the electrodes 2, 2 form an oscillating circuit with the self-inductance iii in which the partial capacitances between I and 2 and between I and 2 are in series. From the coatings 4 and 4', the necessary feedback voltage for the maintenance of the oscillation is fed to the grids l2, l3 of the oscillator tubes 8. 'l, by means of the crossed connection with the grids that is necessary for the generation of the oscillations. The coatings G and 4 obtain their voltage from the dielectric displacement currents originating in the electrodes 2, 2. To prevent those displacement currents from passing to the coatings lying diagonally opposite, a conductive strip 5 is mounted in the center between the coatings 4, 4, and is maintained at zero potential.

i i and I5 represent variable resistances which serve to adjust the grid voltage of grids l2 and i3.

i8 and I! represent protective grids for the anodes of the usual construction. The grid voltages of grids I2 and [3 are fed to the grids of amplifier tubes [8 and [S which work in counterphase, and whose anodes are connected to the resonant circuit which transmits its energy to the antenna 2| by an inductive coupling.

The oscillating circuit thus described is characterized by its complete symmetry, simplicity of arrangement, and by use of only a few connec tions.

Fig. 2 shows a so-called three point connection oi the oscillating circuit, which is also suited for use with a condenser microphone as described in this invention. In this circuit, also, the electrodes 2, 2' are connected with the ends of the self-inductance Hi. The supply of feedback voltage to the tube grid 22 originates only from one of the coatings 4, 4, because only one oscillator tube 23 is provided in this circuit. In this particular circuit the feedback voltage comes from coating 4' which is also connected to the grid 24 of amplifier tube l9, whereas the grid 25 of tube it, which works in counter-phase with tube i3, is connected to the other coating 4, The parts hi and i5 are variable resistances for the adjustment of the grid voltage. In this circuit only three tubes, two resistances and one selfinductance are necessary, so that such a circuit can be produced simply and at very low cost.

The condenser microphones heretofore used have the disadvantage that there must be a high continuous current voltage between the mar.-- brane which is held at zero potential and the electrode lying opposite it. This voltage exerts an electrostatic force of attraction between the membrane and the electrode, so that it is possible that the membrane may touch the electrode and thus produce a short circuit which makes the microphone useless.

This disadvantage can be avoided by using the condenser microphone described in this invention. The circuit shown in Fig. 3 is an example when there is only essentially the tube grid voltage between membrane I and the electrodes 2, 2, which exerts only a negligible force of attraction on the membrane. In this circuit the feedback voltage originates at the electrodes 2, 2 which are cross connected with the grids l2 and 13 of the oscillator tubes 6 and 1 working again in counterphase. The oscillating circuit containing the self-inductance it! is capacitively closed through the partial capacitances of the coatings 1i and A opposite the membrane I. The ends of the selfinductance IE! are connected with the anodes 8 and 9 of the tubes 6 and-l. The parts I4 and I5 are variable resistances for the adjustment of the grid voltage.

Fig. 4 shows an oscillating circuit in which no continuous current voltage is imposed between the membrane l and the electrodes 2, 2. Here a condenser microphone is used in which further conductive plates 25 and 21 are located between the electrodes 2, 2' and the coatings 4 and 4, which are insulated by means of ceramic plates from the electrodes 2, 2' and the coatings 4, 4 as well as from each other. The circuit as shown in Fig. 4 differs from that shown in Fig. 1 only in that the two electrodes 2, 2 are coupled purely capacitively with an oscillating circuit such as the one shown in Fig. 1. The required degree of coupling can easily be achieved by means of a properly chosen dielectric with a high dielectric constant between the electrodes 2, 2 and the plates 26 and 2'5. The amplifying stage connected to the oscillating circuit corresponds in construction exactly to the one shown in Fig. 1.

It may easily be seen that capacitances of any desired value can be created by means of successively stacked ceramic plates or by using the front and back sides of ceramic plates, which is the preferred form shown because of the saving of space, or this may also be done by using only the two electrodes 2, 2' and then inserting ordinary condensers 63, 64 in the circuit. In Fig. 4.- one side is shown using such ordinary condensers B3; 54.

Oscillating circuits of the kind described can be constructed in a very compact manner in con neotion with a condenser microphone of the type disclosed herein.

Fig. 5 shows such a construction in perspective view. The individual parts are so arranged on a baseii that the base with the attached parts fits into a hollow cylinder 30'. which is shown in the drawing only in longitudinal section to allow the exposure of the individual parts. The hollow cylinder is shown in perspective in Fig. 6, but on a smaller scale. The mounting has three compartments 3!, 32, 33 in one of which 31 the tubes 1-245, 36 and one other tube, not shown in the drawing, are situated and mounted parallel to the axis of the housing. Between the tubes are resistances 31 which are also mounted parallel to the axis of the housing. The fastening of the tubes in the illustrated form is accomplished by fastening the bottoms of the tubes by means of the lead wires to insulating bodies in the central compartment 32, while the tops of the tubes are held in place by inserting their pointed ends or tips into the openings 38 in an insulating plate which may be made of pressboard and forms a closure at the end of the hollow cylinder. In the compartment 33 we put the self-inductances so that they are mutually parallel, but perpendicular to the axis of the cylinder. In Fig. 5 only the ends ll of those self-inductances which have the ceramic coatings 42 are shown. By turning the metallic screws 43 one way or the other, the induotances can be adjusted to suitable values. This adjustment can also be made after the hollow cylinder 30 has been slipped over the assembly, because there are openings 44, 45 provided opposite screws 43 for the insertion of tools such as a screw driver (see Fig. 6). In addition, there is another opening 66 in the hollow cylinder which is extended by a slit to the edge of the cylinder. This slit is intended to receive the antenna wire. The center compartment is meant to accommodate essentially the electrical connections and the insulating parts.

The structure of the condenser microphone, which is the object of this invention, is shown in Figs. '7 to 11. Fig. 7 is a longitudinal section through the center aXis of the preferred form of microphone. The holding ring t! carries the insulating body 3 in the form of a ceramic disc glued to it.- The insulating body is a disc with a thickened edge, and the electrodes 2, Z, the coatings 4, i and the metallic strip 5, which is maintained at zero potential, are fastened to it. The substantially semi-circular electrodes 2, 2 lie opposite each other (see Fig. 8). Between them is the metallic strip which prevents the flow of dielectric displacement currents between the electrode 2 and the coating i, and between the electrode 2 and the coating 4, respectively. The electrodes 2 and 2' are hollow to reduce the effect of their mutual capacity (see Fig. 7). They may also be built so that the hollow space gradually enlarges from the top to the bottom.

Above the holding ring 41 is the intermediate ring it, and above the latter there is the microphone cover 39. The bottom of this cover is not perfectly plane or fiat, but has one or more ringshaped ribs 59, opposite which on the intermediate ring are one or more circular recesses or grooves 5i. The membrane 2 is clamped be tween the microphone cover 49 and the intermediate ring 58 so that the membrane is stretched toward the circumference with a uniform force by the elevations 59 and the recesses 5i. Between the holding ring 477 and the intermediate ring 48 a thin spacing ring 52 is inserted, which determines the distance between the membrane i and the surface of the electrodes 2, 2, because when the spacing ring 52 is not inserted, the membrane 5 is in direct contact with the electrodes 2, 2' due to the thickness of the intermediate ring Q8. The distance between membrane l and the surface of the electrodes 2, 2 is generally adjusted to between .007 and .02 mm. There may also be provided a retaining ring 69 for the cover 49 which is fastened on to the outer edge of the intermediate ring l8. However, this retaining ring may also form a part of the cover itself. The individual parts of the microphone are held together by means of screws 53.

The cover 49 of the microphone contains a few circular openings 5 through which the sound waves may reach the membrane I. Part of these openings, for instance those in the center, can be covered up so that the sound waves will not actuate the membrane too strongly in the center, and thus a true reproduction may be obtained by a plate 59 of insulating mate-rial. This insulating plate serves at the same time as insulating means for the antenna wire. Such a plate can, for instance, be fastened on to the screen 55 (see Fig. 5), which serves to prevent the admission of foreign parts to the membrane l and is fastened over the openings 5% in the cover of the microphone.

The electrodes 2, 2 are provided with circular recesses St on their faces opposite the membrane i in order to allow the bypass of an air layer between the membrane i and the electrodes 2, 2, and to prevent a blocking of the motion of the membrane strong air compressions. In order to equalize slow changes in pressure, as may occur through changes in the external atmospheric pressure for instance, a hole 6! is provided.

The modifications of Figs. 10 and 11 differ from that of Fig. '7 mainly in that the ceramic plate is held in place by means of a special ring 56,

instead of by glue. The fastening by the ring 56 is particularly suitable when the frequency changes of the transmitter due to changes in temperature must be compensated for. Depending on the requirements, ceramic plates 3 with elevated edge, recessed edge, or plane edge can be used for this purpose. Fig. '7 shows a ceramic plate with an elevated edge 51, Fig. 10 shows one with a plane edge, and Fig. 11 one with a recessed edge 53. By means of these different shapes of the ceramic plate 3 we can give the heat expansion of the ceramic plate 3 a variable value, calculated from the clamping plane and from the electrodes. The differences are determined by the component of the expansion caused by the ceramic parts. In all cases it is possible to shape the edge of the ceramic plate so that the changes in capacity due to changes in temperature affecting the distance between membrane and electrodes 2, 2' are compensated for.

The oscillating circuit, as shown in Fig. 5, can be inserted into a tube together with the necessary batteries. Fig. 12 shows such a construction, with a suitable tube 65 which consists of two parts, on one end of which there is the switch 65 and on the other end the insulated terminal for the antenna connection.

While we have described herein some embodiments of our invention, we wish it to be understood that we do not intend to limit ourselves thereby except within the scope of the claims hereto or hereinafter appended.

We claim:

1. In a frequency modulated oscillating circuit, a condenser microphone having a diaphragm, a plate of dielectric material substantially parallel thereto, at least two symmetrically arranged electrodes on the face of the plate nearest the diaphragm forming with said diaphragm two capacitances of said circuit, and electrically conductive coating sections on the other face of the plate opposite said electrodes and forming therewith capacitances in said circuit.

2. In a circuit as claimed in claim 1, a strip of conductive material between said sections, and means to maintain said strip at zero potential.

3. In a frequency modulated oscillating circuit which includes a self-inductance and two oscillator tubes having anodes and grids and operating in counter-phase relation, a condenser microphone having a diaphragm and at least two symmetrically arranged electrodes forming with said diaphragm two capacitances, a plate on the opposite side of the electrodes from the diaphragm, electrically conductive coating sections on the opposite face of the plate from and opposite said electrodes and forming therewith capacitances, connections from said electrodes to the ends of said self-inductance and to the anodes of said tubes, and cross connections from said sections to the grids of said tubes.

l. In a frequency modulated oscillating circuit which includes a self-inductance, an oscillator tube having an anode and a grid and two amplifier tubes having grids and operating in counter-phase relation, a condenser microphone having a diaphragm and at least two symmetrically arranged electrodes forming with said diaphragm two capacitances, a plate on the opposite side of the electrodes from the diaphragm, electrically conductive coating sections on the opposite face of the plate from and opposite said electrodes and forming therewith capaci- 11 tances, connections from said electrodes to the ends of said self-inductance, a connection from one of said electrodes to the anode of said oscillator tube and from one of said sections to the grid thereof, and connections from said sections to the grids of said amplifier tubes.

5. In a frequency modulated oscillating circuit which includes a self-inductance and two oscillator tubes operating in counter-phase relation, said tubes having anodes and grids a condenser microphone having a diaphragm and at least two symmetrically arranged electrodes forming with said diaphragm two capacitances, a plate on the opposite side of the electrodes from the diaphragm, electrically conductive coating sections on the opposite face of the plate from and opposite said electrodes and forming therewith capacitances, cross connections from said electrodes to the grids of said tubes, and connections from said sections to the ends the sshinductance and to the anodes of the tubes.

6. In combination, a hollow cylinder, an insulating plate closing one end of the cylinder, a microphone having a diaphragm closing the other end of the cylinder, a frequency modified oscillating circuit arranged within the cylinder,

microphone having at least two symmetricaliy arranged electrodes forming with said diaphragm two capacitances of said circuit, in which said circuit includes tubes, resistances and a selfinductance, means dividing the interior of the cylinder into three compartments, the compartment adjacent one end containing the tubes and resistances with the tubes extending parallel to the axis of the cylinder, the compartment adjacent the other end containing the self-inductance arranged transversely to the axis of the cylinder, and the intermediate compartment contains the electrical connections of the circuit.

7. In a device as claimed in claim 6, in which the tubes are located in the compartment adjacent the insulating plate, said plate having holes therein in which the tips of the tubes are held, and said electrical connections including wires connected to the bases of the tubes and holding them in position.

8. In a device as claimed in claim '7, said cylinder having an opening into the compartment containing the self-inductance, and the self-inductance having an adjusting means opposite such opening.

9. A device as claimed in claim 8 having an opening from the outside into the space below the diaphragm.

10. In a device as claimed in claim 9, a protective mesh cover above said diaphragm, and a plate secured in the center of the mesh covering a part of the openings therein.

11. In combination with a device as claimed in claim 10, a cylindrical container enclosing such device and batteries therefor, a switch on one end of the container and an antenna terminal on the other end thereof.

12. A condenser microphone having a diaphragm, a plate of dielectric mater'el substantially parallel thereto, at least two symmetrically arranged electrodes on the face of the plate nearest the diaphragm forming therewith two capacitances, and electrically conductive coating sections on the other face of the plate opposite said electrodes and forming therewith capacitances.

13. In a microphone as claimed in claim 12, a strip of conductive material between said sections.

14. A condenser microphone ha 13; a holding ring, a protective cover, a disc of insulating material. held by said ring, an intermediate ring between the cover and the holding ring, a diaphragm clamped between the intermediate ring and the cover, and symmetrical electrodes on the side of the disc facing the diaphragm and forming capacitance-s therewith.

15. In a microphone as claimed in claim i l, a spacing ring between said holding and intermediate rings.

15. In a. microphone as claimed claim 1.4, said intermediate ring and cover having mating projections and recesses between which the die. phragm is clamped.

17. In a microphone as claimed in claim 14, said plate having a low dielectric constant.

18. In a microphone as claimed in claim 14, said electrodes having recesses in the side facing the diaphragm.

19. In a microphone as claimed in claim 14, threaded means securing said disc to said holding ring, the material, proportions and spacing of the parts being such that upon changes in temperature the distance between the diaphragm and the electrodes varies in such a way that the frequency of the device remains substantially constant.

References ilited in the file of this patent UNITED STATES PATENTS Number Name Date 1,622,039 Lee Mar. 22, 1927 1,631,583 Depew Juno '7, 192'! 2,008,713 Hayes I July 23, 1985 2,311,491 Turner Feb. 16, 1943 2,386,049 Hausz Oct. 2, 1945 FOREIGN PATENTS Number Country Date 122,188 Australia Sept. 4, 1946 

